Porous, extensively fibrillated polytetrafluoroethylene and method of preparing same



Oct. 22, 1968 H. P. LANDI 3, ,2

POROUS, EXTENSIVELY FIBRILLATED POLYTETRAFLUOROETHYLENE AND METHOD OFPREPARING SAME Filed Feb. 28, 1966 INVENTOR. 1 t HENRY PATRICK LA/VD/ ATTORNEY United States Patent 3,407,249 'POROUS, EXTENSIVELY FIBRILLATEDPOLY- TETRAFLUOROETHYLENE AND METHOD OF PREPARING SAME Henry PatrickLandi, Yorktown Heights, N.Y., assignor to American 'Cyanamid Company,Stamford, Conn., a corporation of Maine Continuation-impart ofapplication Ser. No. 467,987, June 29, 1965. This application Feb. 28,1966, Ser. No. 530,346

5 Claims. (Cl. 264-49) This applicaton is a continuation-in-part of acopending application for Letters Patent, Ser. No. 467,987, filed June29, 1965, and now abandoned.

The present invention relates to unsintered, porous, completelyfibrillated polytetralluoroethylene sheets and to methods for theirpreparation. More particularly, the invention relates to a modificationof normally hydrophobic sheets of polytetrafluoroethylene and to preparetherefrom wettable, unsintered, porous and completely fibrillatedstructures eminently suitable for a variety of diverse uses, forinstance, as matrices for fuel cells, as gas permeable-electrolyteliquid impermeable backings for fuel cell electrodes, as batteryseparators, as membranes for desalinization or as ordinary filteringaids.

It is well known that porous polytetrafluoroethylene sheets are highlyhydrophobic. As such, they cannot normally be employed in applicationswhich require the wetting of their surfaces. If sheets of hydrophobicpolytetrafluoroethylene can be sufficiently modified so as to permitboth the suitable wetting of their surfaces and their internal volume,sundry uses presently unavailable the invention, an aqueous dispersionof polytetrafluoroethylene or Teflon particles which the approximately0.1 micron to 0.5 micron in diameter with wrinkled surfaces resemblingdistorted, collapsed, deflated and hollow spheres as viewed under anelectron microscope, is admixed with a solvent-extractable, highlyviscous resinous polymer, such as, for instance, moltenpolymethylmethacrylate, polyethylene oxide or an alkali metalshellacate, rosinate or tallate or any equivalent thereof, and,optionally, if desired, with sufiicient solvent-insoluble non-electronconducting inorganic or organic filler which can constitute as much as98% by weight of the finished sheet. Shearing of the Teflon particlesoccurs when the latter components of the over-all composition areconventionally hot-milled and extruded. The milling and extrudingoperations are critical, for otherwise the sphericalpolytetrafluoroethylene latex particles are not transformed into anetwork of lengthy, randomly-oriented fibers. Subsequent to extrusion, asheet-like structure is formed. The latter is then soaked in a suitableselective organic solvent to dissolve and thereby remove from the sheetall of the solvent-extractable viscous, resinous polymeric additive.So-treated sheet is washed with water and dried.

In an alternative embodiment, a highly porous, unsintered, extensivelyfibrillated polytetrafluoroethylene sheet devoid of inorganic or organicfiller is formed in accordance with the practice of the invention. Thissheet is next treated with a solution containing a non-electronconducting filler as by dipping the formed sheet into the lattersolution to effect impregnation of that sheet with filler.

In general, any commercially-available aqueous disper- "ice sion ofpolytetrafiuoroethylene in varying amounts may be treated in the processof the invention. For instance, one such aqueous dispersion employedcontains from 59% to 61% solids or particles of polytetrafiuoroethyleneand from 5.5% to 6.5% of a commercially available non-ionic wettingagent, namely, either an octylphenol polyoxyethylene or a nonylphenolpolyoxyethylene, based on the weight of the particles characterized asranging from about 0.1 micron to 0.5 micron in diameter. The dispersionin an amount ranging from about 1% to about 40%, by weight based onpolytetrafl-uoroethylene solids contained in said dispersion, is thenadmixed with, as by blending, a solvent-extractable, highly viscousresinous polymer, hereinabove exemplified, in amounts ranging from about98% to about 40%, based on the overall solids mixture. Where a filledstructure is desired, there may be incorporated into the blended mixturea solventinsoluble, inorganic or organic non-electron conducting fillerin amounts ranging from about 10% to about 50%, based on the overallsolids content. However, if desired, the filler may be added even afterthe sheet has been formed as by vacuum impregnation.

The above mixture is intimately blended and milled at temperaturesranging from about C. to about 200 C. and extruded as by injectionmolding or calendering. A sheet-like structure is formed. The latter isnext treated with any organic solvent in which the above extractableresin component is soluble. Illustrative solvents include: acetone,methyl ethyl ketone and their equivalents, Matted sheets are desirablyprepared preferably by rolling the solvent-extracted sheets.

So-prepared sheets when immersed in an alcohol bath can be readilywetted by electrolytes normally employed in fuel cell matrices. Toeffect exceptional wetting, the polytetrafluoroethylene sheet can bepreferably contacted with small amounts, usually from 0.01% to about 2%,of a suitable water-soluble salt of fiuorinated aliphatic surfactants,such as ammoniumor sodium perfluorocaprylate.

As hereinabove stated, it is an advantage of the invention thatsubstantially non-electron conducting inorganic or organic fillers ofthe inorganic and organic types, can be readily incorporated within thenetwork of unsintered polytetrafiuoroethylene fibers to levels as highas 98%, by weight, of the over-all final structure. Such non-electronconductive fillers which are defined herein as those which do notpossess metallic conductivity are, for instance, metallic oxides, suchas tantalum oxide and ceric oxide; water-insoluble salts, such as bariumsulfate, calcium sulfate, calcium phosphate, zirconium phosphate andzinc phosphate; ion-exchange resins, such as tin acid phosphate andsulfonated polystyrenes; inert thermoplastic polymers, such asalkali-etched polytetrafiuoroethylene fioc,polymonochlorotrifiuoroethylene, polyformaldehyde and polypropyleneoxide.

Advantageously, it is within the purview of the instant invention toutilize a formed sheet as the matrix component of a unitary, laminatedcomposite by compressing, for instance, the formed sheet of theinvention positioned between carbon-filled, platinized electrodes.

In order to further clarify the invention, these and other embodimentsthereof are shown in the accompanying drawing and will be described indetail in conjunction with said drawing, in which:

FIGURE 1 is an exploded plan view, partially in section, of a fuel cellemploying the matrix of the present invention, and

FIGURE 2 is a partially expanded side View, partially in section, of thefuel cell of FIGURE 1.

In FIGURE 1, a porous, unsintered, extensively fibrillatedpolytetrafiuoroethylene matrix 1 as prepared in Example. 2 below ispositioned between a fuel electrode 2, such as platinum, and an oxygenelectrode 3, such as platinum, palladium or silver. Abutting the latterelectrodes are current collector screens 4 and 5 which comprise nickelor other suitable inert metal. Nickel wire mesh spacers 6 and 7 areemployed to compress the collector screens against the electrodesproviding for better contact between screen and electrode as well aselectrode and membrane. The wire mesh spacers are positioned exteriorlyto the current collectors. To the outside of the spacers are gaskets 8and 9 of any suitable material, such as polytetrafiuoroethylene orsilicone rubber. These gaskets seal as well as separate the chamberscontaining reactants. Exterior to the gaskets are housing members 10 and11 to which are attached thermocouple 12 and heat control probe 13 andhaving inlet stainless steel or other inert metal tubing 14 and 15through which hydrogen and oxygen as the illustrative fuel and oxidant,respectively, are separately introduced into the fuel cell. Nickeltubing 16 and 17 are provided as vents for unused gases. Wire leads 18and 19, connected to current collector screens 4 and 5, are theconductive members through which current flows from and to the fuel cellthrough the external circuit when the fuel cell is in operation. Thecell, secured by means of bolts 20 and nuts 21 as shown in FIGURE 2, canbe heated, if desired, by, for instance, an external electrical heatingpad 22. The temperature of the cell, determined by the thermocouple 12,is controlled by heat control probe 13.

A matrix and its performance in fuel cells can be conveniently preparedin the following illustrative examples which are not to be taken aslimitative of the invention. Unless otherwise stated, the parts givenare by weight.

Example 1 A blend of (a) 20%, by weight, of polytetrafiuoroethylene inthe form of an aqueous dispersion containing 59%6l%polytetrafiuoroethylene solids and 5.5%- 6.5%, by weight based on weightof said solids, of an octylphenolpolyoxyethylene and (b) ofpolymethylmethacrylate, by weight based on the overall solids, is milledon preheated rolls at 170 C.-175 C. During the milling operation, thepolytetrafiuoroethylene particles form lengthy interwoven,interconnected fibrils or fibers. A /8" x 2" x 4" plaque is next formedby injection molding the above blend and compressing the same betweencaul plates for from 5 to 10 minutes at 160 C.-170 C. and 3000 p.s.i.g.This plaque is cooled to room temperature and released from the mold. I

The formed sheet measures approximately eight inches in diameter andfrom ten to twenty mils in thickness. The sheet is soaked several timesin acetone to dissolve the polymethylmethacrylate present in the sheet.It is then rinsed with alcohol, washed several times with deionizedwater and, finally, dried by rolling between blotter paper.

Example 2 Performance of a fuel cell is illustrated in this example.

A hydrogen-oxygen fuel cell as hereinabove described is operated at 150C. with phosphoric acid electrolyte immobilized in the matrix of highlyporous, unsintered, completely fibrillated polytetrafiuoroethylene sheetas prepared in Example 1 above.

The hydrogen and oxygen electrodes herein employed comprise a 50 meshtantalum screen to which is applied a mixture of 9 mgs. of platinumblack per square centimeter of screensurface in polytetrafiuoroethylenewaterproofing binder. The area of each electrode exposed to theelectrolyte is five square centimeters.

Extensively fibrillated, porous, unsintered polytetrafiuoroethylenematrix is initially treated for utilization in a fuel cell by'soakingthe latter' for several hours in 85% phosphoric acid containing 0.1%ammonium perfiuoroca'prylate at C. C. and allowing the matrix to coolwhile still immersed in the electrolyte.

The wet matrix is then incorporated into the hereinabove illustratedhydrogen-oxygen fuel cell and the observed current-voltage relationshipis tabulated in Table I below as follows:

TABLE 1 Current (ma/emi Operating voltage (volt) O 0.989

The internal resistance of the cell measures 0.44 ohm at C.

Example 3 The hereinabove-described hydrogen-oxygen fuel cell isoperated at 70 C. utilizing 6 N aqueous potassium hydroxide electrolyteimmobilized in the matrix as prepared in Example 1, above.

Each of the electrodes which sandwich the matrix contains 9 mg./cm.platinum black mixed with polytetrafiuoroethylene binder on a 100 meshnickel screen.

The completely fibrillated, porous, unsintered polytetrafiuoroethylenematrix as prepared in Example 1 is further treated by soaking the matrixin ethanol and then displacing the alcohol with 6 N aqueous potassiumhydroxide by soaking for about one hour. The wet matrix is thenincorporated into the hydrogen-oxygen fuel cell as hereinabove describedand operated continuously for three days at 100 ma./cm. At the end ofthis period, the operating cell voltage is 0.855 volt at 100milliamperes per square centimeter (ma/cm?) and an internal resistanceof 0.03 ohm Example 4 Polymethylmethacrylate (95 parts) is heated andmilled to a molten viscous state on a rubber mill maintained at atemperature between C. and 190 C. To the rubber mill are next added 5parts of polytetrafiuoroethylene in the form of a 60% aqueous emulsionand 95 parts of ceric oxide are blended into the moltenpolymethylmethacrylate. On cooling the blend, resultant solid is groundinto pellets and injection molded into a A; inch x 2 inch x 4 inchplaque. The plaque is next compression molded at temperatures between180 C. and 200 C. at a pressure of about 1000 p.s.i. into a sheet 30mils thick and 6 inches square. This sheet is then immersed in acetonefor sixteen hours at 25 C. so as to extract polymethylmethacrylatetherefrom. Thereafter, the sheet is washed with acetone for from one totwo hours in subsequent washings.

Resultant sheet is then washed with ethyl alcohol and water. After allthe polymethylmethacrylate is removed, the sheet is composedsubstantially of 95% of ceric oxide and 5% of completely fibrillatedpolytetrafiuoroethylene. The sheet is then completely saturated withwater which displaces the ethyl alcohol. The sheet is next immersed in a30% aqueous potassium hydroxide electrolyte solution until com letelysaturated.

Good performance of an hydrogen-oxygen fuel cell is noted when the aboveprepared matrix is utilized at temperatures even as high as 200 C.

Example 5 Following the procedure of Example 4 above in every detail,except that the'sheet will contain 95%, by weight, of tin acid phosphatein lieu of 95% ceric oxide. The soformed sheet is saturated with waterin the same manner and, subsequently, is vacuum impregnated with 100%phosphoric acid until saturated.

Resultant impregnated sheet is then cutthree inches square and placedbetween 2 inch square electrodes defined in Example 3. Thecompressedlaminate is inserted in a hydrogen/oxygen fuel cell which isoperated at C. with good attendant performance. Current density andoperating voltage data are recorded in Table II below.

TABLE I Operating voltage (in volts): Current density, ma./cm. 0.970Open circuit 0.927

The internal resistance of the cell measures 0.014 ohm at 175 C.

Substituting for the tin acid phosphate in the above example asodium-etched polytetrafiuoroethylene floc, there is prepared a matrixpossessing similarly enhanced performance characteristics.

Example 6 A sheet containing 80% by weight of barium sulfate and byweight of unsintered, completely fibrillated polytetrafiuoroethylene isprepared as described above in Example 4. The porous, non-electronconducting filled 6 with and without filler prepared in accordance withthe procedure set forth in Example 4 above.

typical filled and unfilled Sheets A through F above are summarized inTable V below.

TABLE V Porosity Tensile Properties Volume percent of pores TensileSheet Total Porosity within the range of- Modulus Percent (Vol. 01 Elas-Elongation tieity, to break 0.035}L 0.1-1.0 1.040.0 4 p.51.

2 At yield.

sheet is then washed successively with alcohol, and then with water and,finally, is saturated with 100% phosphoric acid. Resultant sheet is cutto size and placed between two standard platinized carbon water-proofedelectrodes and tested as a hydrogen-air fuel cell at 175 C. with goodperformance. The data is summarized in Table III below.

TABLE III Operating voltage: Current density, ma./cm.

0.966 Open circuit 0.885 10 The internal resistance of the cell measures0.059 ohm at 175 C.

Example 7 It can be readily seen from the above table that the highlyporous, completely fibrillated, unsintered poly tetrafiuoroethylene ofthe invention functions as an inert binder or support for largequantities of a variety of nonelectron conducting fillers.Advantageously, enhanced structures which exhibit unexpected goodtensile strength properties are thereby obtained.

I claim:

1. A method for preparing a porous, unsintered, completely fibrillatedpolytetrafiuoroethylene sheet which comprises: intimately blending fromabout 1% to about 40%, by weight, of polytetrafiuoroethylene in the formof an aqueous dispersion and from about 98% to 40%, by weight, of anorganic-solvent extractable viscous polymer, based on the weight of theoverall mixture; milling the latter mixture at temperatures ranging fromabout C. to about 200 C.; extruding the milled product to form a sheet;immersing said formed sheet in an organic solvent selective for thesolvent extractable viscous polymer thereby dissolving the latterpolymer from said formed sheet; and recovering the latter as a porous,unsintered, completely fibrillated polytetrafiuoroethylene sheet.

2. A process according to claim 1, in which the organicsolventextractable polymer is polymethylmethacrylate.

3. A process according to claim 1, in which the organic solvent isacetone.

4. A process according to claim 1, in which there is incorporated intothe mixture to be blended from about 10% to about 50% of a. nonelectronconducting filler insoluble in said organic solvent selective for theextractable viscous polymer.

5. A process according to claim 1, in which the formed sheet is furtherwashed with alcohol and subjected to the action of an electrolyte.

8 Stand 26449 XR Goldsmith 26449 Gore 264127 XR Rasmussen 161169 JULIUSFROME, Primary Examiner.

PHILIP E. ANDERSON, Assistant Examiner.

1. A METHOD FOR PREPARING A POROUS, UNSINTERED, COMPLETELY FIBRILLATEDPOLYTETRAFLUOROETHYLENE SHEET WHICH COMPRISES: INTIMATELY BLENDING FROMABOUT 1% TO ABOUT 40%, BY WEIGHT, OF POLYTETRAFLUOROETHYLENE IN THE FORMOF AN AQUEOUS DISPERSION AND FROM ABOUT 98% TO 40%, BY WEIGHT, OF ANORGANIC-SOLVENT EXTRACTABLE VISCOUS POLYMER, BASED ON THE WEIGHT OF THEOVERALL MIXTURE; MILLING THE LATTER MIXTURE AT TEMPERATURES RANGING FROMABOUT 170*C. TO ABOUT 200*C.; EXTRUDING THE MILLED PRODUCT TO FROM ASHEET; IMMERSING SAID FORMED SHEET IN AN ORGANIC SOLVENT SELECTIVE FORTHE SOLVENT EXTRACTABLE VISCOUS POLYMER THEREBY DISSOLVING THE LATTERPOLYMER FROM SAID FORMED SHEET; AND RECOVERING THE LATTER AS A POROUS,UNSINTERED, COMPLETELY FIBRILLATED POLYTETRAFLUOROETHYLENE SHEET.